Distillation purification of phthalic anhydride



Feb. 23, 1954 H. J. wEsT ET AL 2,670,325

DIsTILLATIoN PURIFICATION oF PHTHALIC ANHYDRIDE Filed July 14, 1951 ,wf-,4495 c07- car awa/1? a 5//0 as 25 Patented Feb. 23, n1954 DISTILLATION PURIFICATION 0F PHTHALIC ANHYDRIDE Herbert J. West, Pittsburgh, and Kenneth Goodemoot, Bridgevlle, Pa., assigner: to American Cyanainid .CompanL'NeW York, Y., ageorporation of Maine Applicata@ July i4, 1951, serial No. 235.125 3 Claims. (Q1. v20g-57) This invention relates to the purification of crude phthalic kanhydride by a combined heat treatment and fractional distillation procedure.

In the commercial production of pure phthalic anhydride it was previously the practice to subject the crude condensate from the catalytic .converters to two separate distillation procedures. The phthalic anhydride converter product, containing color-forxning and odorgforming impurities, was first heat treated for some 16 to 24 Vhours and was then subjected to a total distillation. The distillate was then revaporized ,and the vapors were subjected either to fractionation or tol fractional condensation to produce a phthalicanhydivide product of commercially acceptablepurity.

Our present invention is directed to a modification of this procedure in which the preliminary heat treatment is greatly shortened and the intermediate total distillation isentirely eliminated. We have found that when the heat treatment is carried out in the presence of very small amounts of alkali metal phthalates, which are hereinafter described as catalytic quantities, several unexpected advantages are obtained. In the rst place, the heat treatment at approximately the boiling point of phthalic anhydride (about 28o-285 C.) can be reduced to about 1/3 to l@ of its former duration. Secondly, the presence of alkali metal phthalate in catalytic amounts obviates the necessity of an intermediate total distillation if a heads cut is taken; in other words, the mixture obtained by heat treatment in the presence of the alkali metal phthalate can be subiected directly to a fractional distillation provided the benzoic acid contant of the converter `product is removed as a heads cut.

odor-forming impurities in the converter prodd uct vary with the particular hydrocarbon employed; when naphthalene is oxidized over tired catalyst at temperatures of 40o-5009 G. or. higher theV resulting gases contain .alohazianhthocinone but have` a, considerably less contentof'malcic auf Percent Percent Percent Hydrocarbon Used Buzgc Lcleic APhthh l ci ci nhy. ri e Naphthaleue 1 0. 03 0. 4 99. 7 Orthoxylene. Y.. .1 0. 4 1. 7 96. Q

l Presumably toluic acid.

In accordance with the present invention, the heat treatment of either of these crudes is carried out in the presence of catalytic quantities of alkali xnetalphthalate within the range of approximately 0.02% to 0.2%, based on the weight .of the crude. By alkali metal phthalate we mean, of course, a phthalic acid sait of an alkali metal: i. e.. lithium, sodium, potassium, etc. This alkali metal phthalate may be added, as such, directly to the crude phthalic converter product before beginning the heat treatment. We prefer, however. to add an' alkali metal alkali, such as the hydroxide or carbonate,.in amounts. of about 0.02% to 0.1% to .the crude phthalic anhydride as an agueous solution before beginning the heat treatment, which results 'in forming the alkali metal phthalate in situ. Y The. timeof'the heat treatment must be governed largely by the amount and type of impurities present in the phthalic anhydride, In general" its duration is on the order of about 342 hours.' We iind,'however that with any given crude the prsence or. catelytl quantities only. of alkali metal phthalate reduces thev tlzne'o'f heat treatment to from Vi. to l/acf that which would be necessary if` the catalyst were not present. Thus, for example, a. converter product obtained by the catalytic air oxidation of naphthalene over a ixcd vanadium oxide catalyst at tempel autresl of 425y50ll G. waslieabtreated after addice' '910.5% ef feium-hvdiitida instanties were .withdrawn after 8f me .16: .beurs cf: henkie en@ iotbe'rl sample oithe :une product Iwas heated for 3 17 hours in the absence of a condensation catalyst. The samples were then subjected to vacuum distillation. The results were as follows:

results show that the use of 0.05% of caustieredueed the heat treating time requirements tov less than 1%, since Athe color and heat tests of the product obtained by reiluxing the crude with sodium phthalate for 8 hours were superior to those of the productohtainedfrom the crude reduxed for 17 hours without the catalyst. i

A heads cut, which usuallyamounts tov between 1% and 5% of the charge, maybe taken either during or after the heat treatment, and is preferably though not necessarily taken oif at {atmospheric pressure. We have' found that a heads cut vat this pointremoves volatile color-forming impurities and also reduces the content of `henzoic acid in the iinal purified phthalic anhydride to a very low figure. That most of the benzoic acid in the crude converter product is removed inthe heads cut is shown by our-analysis-fgures; thus, from a naphthalene converter product heated for 12 hours at 280 C. with 0.05% sodium hydroxide aheads out of 1.8% contained 2 -3% of benzoic acid and had a color of 175; an orthoxylene converter product similarly treated produced a heads cut of 2.8% containing' almost 25 of benzoic acid and hada color of 4 00. y It will thus be seen that this color removal and benzoic acid removal is an important feature of our invention. "l

AS has been stated, the heat condensation of impurities the presence of catalytic quantities only of alkali metal phthalate is so effective that the product can be subjected immediately to a iinal distillation for the' production of a commercially pure White phthalic anhydride.' In other words, the formerly practiced procedure vof lsub'- .ecting the heatf-treated phthalicV anhydride to total distillationfollowedpby fractionation o f the distillate need no longer b e used; the total distillation step can be entirely omitted. Fractionation of the heat-treated product canbc carried out either at atmospheric pressure or under a partial vacuum; of these, a vacuum distillation at about mm. of mercury absolute pressure is preferred. Excellent results are obtained With a '1`:1 reflux ratio using a column ccu'itairling from 4 to theoretical plates. The values shown on the drawing were obtained from a nailhthalene converter 4prod'- uct, having the characteristics of" that shown above, by such a vacuum, distillation; lt'V will be understood, however, that 'diiferf'vnt .results are obtained with other crudes; A i l It is an important advantage of the present invention that the quantities of residue from the fractional distillation are no greater l'than those produced by heatftreatmen YVinthe absence of condensin I`agex'its.ffiviorefgve lage proportion ofthe `'phtli'alie anhydride 'ntentoffthesedistil: 'lation residues canb frecovc'are`d:in also-calleddl :of only 2.13% as 4 coke exhauster. This is a simple still wherein the residues are distilled at higher temperatures or lower pressures, or both, than are practical in the fractionation step.

The invention will be further described by the following specic examples. It should be understood, however, .that although these examples may describe certain features of the invention in detail, they are given primarily for purposes of illustration and the invention in its broader aspects is not limited thereto.

Eample 1 A converter product from the catalytic oxidation of naphthalene over a xed vanadium oxide catalyst at 425500 C. Was condensed in an oilregulated condenser as described in U. S. Patent No. 2,455,314. A sample of this product was melted and dehydrated and 0.05% by Weight of sodium hydroxide was added as a 50% solution, after which refluxingwas continued to a liquid temperature of 288 C. The total reflux was then changed to a 6:1 reiiux ratio and an overhead fraction was removed until the vapor temperature was 281.5" C., which required about 5 minutes. This procedure was repeatedwith periodic removals of fractions relatively rich in benzoic acid for a total of 5 hours during which 4.4% of the original charge was taken oi as distillate. vThis cut had an APHA color of and contained 2.7% of benzoic acid. The nal liquid and vapor temperatures were 289 C. and 283 C., respectively.

A portion'of the charge weighing 1913 grams was then transferred to a still equipped with an agitator and a 2.5-inch by 20-inch packed column and subjected to vacuum fractionation at an absolute pressure of 30 mm. of mercury and with a 1:1 reflux ratio With the .following'operating conditions and results:

The heat stability of the products was determined by subjecting a sample of each cut to a standard heat test` This test is carried out by heating the material in a sealed tube for 24 hours vat C. and determining its APHA color. Each of cuts 2 and 3 had a color of 10 after heating; the color of cut 1 Was 40.

- The distillation residue, weighing 134 grams, contained'31.8% of non-volatiles; -i..e., material that could not be recovered as phthalic anhydride in a coke exhauster. This corresponds to a loss non-volatiles on the original charge.

Example 2 1 Another. portion of thejcharge of Examplel, following removal of the heads-cut, Wasdistilled the: s'amei fractionation' .equipment at atmospheric pressure `using alii-reflux lratio. The

Temp., O. Distillatc Time l R emarks Liquid 'Vapor Grams Color l 11:20.... 291 278 Ohm-ge weighs l, Ograms. M22-5 1-. 291 278 35 35 55 Cut l. l". P

120.4o C 1:00...` 292 279.5 937 10 15 Cut 2. l l

1I}0.8 C 2:25 295 280 650 5 l() Cut 3.

The distillation residue weighed 330 and contained 11.6% non-volatiles, this heini; 2% on the Weight of the charge.

These results show that high yields of a pure and almost completely colorless phthalic anhydride are obtained with redux time of only ve hours when 0.05% of sodium hydroxide or the corresponding amount of sodium phthalate is added. Similar results are obtained with other quantities Within the range of 0.01% to 0.1% by Weight, although the optimum times of heat treatment may be diierent. It should be noted particularly that the purity and color of the phthalic anhydride of cuts 2 and 3 of both examples is equal to or better than that of the phthalic anhydride of commerce. The process of the present invention therefore produces a commercially acceptable product While reducing the heat treating time and eliminating entirely the total distillation that has heretofore been used.

What we claim is:

1. A method of removing color-forming and odor-forming impurities from crude undistilled phthalic anhydride converter product containing said impurities which comprises heating said undistilled phthalic anhydride converter product at approximately the boiling point thereof for about 3 to 12 hours in the presence of catalytic quantities only of alkali metal phthalate and subjecting the resulting mixture to a fractional distillation including the separate removal of a heads cut relatively rich in benzoic acid, said distillation including passing the vapors of said mixture upwardly in countercurrent contact with a reux of liquid phthalic anhydride and Withdrawing a portion of the overhead distillate so treated as a main cut composed of commercially pure phthalic anhydride.

2. A method according to claim 1 in which the alkali metal phthalate is formed in situ by adding 0.01% to 0.1% of alkali metal hydroxide to the crude phthalic anhydride.

3. A method according to claim 1 in which the fractional distillation is carried out at about 30 millimeters of mercury absolute pressure.

HERBERT J. WEST. KENNETH GOODEMOOT.

Name Date Cooper Jan. 26, 1943 Number 

1. A METHOD OF REMOVING COLOR-FORMING AND ODOR-FORMING IMPURITIES FROM CRUDE UNDISTILLED PHTHALIC ANHYDRIDE CONVERTER PRODUCT CONTAINING SAID IMPURITIES WHICH COMPRISES HEATING SAID UNDISTILLED PHTHALIC ANHYDRIDE CONVERTER PRODUCT AT APPROXIMATELY THE BOILING POINT THEREOF FOR ABOUT 3 TO 12 HOURS IN THE PRESENCE OF CATALYTIC QUANTITIES ONLY OF ALKALI METAL PHTHALATE AND SUBJECTING THE RESULTING MIXTURE TO A FRACTIONAL DISTILLATION INCLUDING THE SEPARATE REMOVAL OF A HEADS CUT RELATIVELY RICH IN BENZOIC ACID, SAID DISTILLATION INCLUDING PASSING THE VAPORS OF SAID MIXTURE UPWARDLY IN COUNTERCURRENT CONTACT WITH A REFLUX OF LIQUID PHTHALIC ANHYDRIDE AND WITHDRAWING A PORTION OF THE OVERHEAD DISTILLATE SO TREATED AS A MAIN CUT COMPOSED OF COMMERCIALLY PURE PHTHALIC ANHYDRIDE. 